Mechanical couplings for joining pipe elements together end-to-end comprise interconnectable segments that are positionable circumferentially surrounding the end portions of co-axially aligned pipe elements. The term “pipe element” is used herein to describe any pipe-like item or component having a pipe like form. Pipe elements include pipe stock, pipe fittings such as elbows, caps and tees as well as fluid control components such as valves, reducers, strainers, restrictors, pressure regulators and the like.
Each mechanical coupling segment comprises a housing having arcuate surface regions which are disposed to engage plain end pipe elements or circumferential grooves that extend around each of the pipe elements to be joined. Engagement between the surface regions and the pipe elements provides mechanical restraint to the joint and ensures that the pipe elements remain coupled even under high internal pressure and external forces. The housings define an annular channel that receives a gasket or seal, typically an elastomeric ring which engages the ends of each pipe element and cooperates with the segments to provide a fluid tight seal. The segments have connection members, typically in the form of lugs which project outwardly from the housings. The lugs are adapted to receive fasteners, such as nuts and bolts, which are adjustably tightenable to draw the segments toward one another and engage the surface regions with the outer surfaces of the pipe elements.
To ensure a good fit between the couplings and the pipe elements, the surface regions on prior art couplings have a single radius of curvature that is substantially matched to the radius of curvature of the outer surface of the pipe element that it is intended to engage. For couplings used with grooved pipe elements, the radii of curvature of the surface regions are smaller than the radii of curvature of the outer surfaces of the pipe elements outside of the grooves so that the surface regions fit within and engage the grooves properly.
This geometrical relation between the surface regions of the coupling segments and the outer surfaces of the pipe elements in prior art couplings results in a tedious and time consuming installation process when mechanical couplings are used. Typically, the coupling is received by the technician with the segments bolted together and the ring seal captured within the segments' channels. The technician first disassembles the coupling by unbolting it, removes the ring seal, lubricates it (if not pre-lubricated) and places it around the ends of the pipe elements to be joined. Installation of the ring seal requires that it be lubricated and stretched to accommodate the pipe elements, an often difficult and messy task, as the ring seal is usually stiff and the lubrication makes manual manipulation of the seal difficult. With the ring seal in place on both pipe elements, the segments are then placed one at a time straddling the ends of the pipe elements and capturing the ring seal against them. During placement, the segments engage the seal, the surface regions are aligned with the grooves, the bolts are inserted through the lugs, the nuts are threaded onto the bolts and tightened, drawing the coupling segments toward one another, compressing the seal and engaging the surface regions within the grooves.
As evident from the previous description, installation of mechanical pipe couplings according to the prior art requires that the technician typically handle at least seven individual piece parts (and more when the coupling has more than two segments), and must totally disassemble and reassemble the coupling. Significant time, effort and expense would be saved if the technician could install a mechanical pipe coupling without first totally disassembling it and then reassembling it, piece-by-piece.
Furthermore, there is a class of pipe coupling segments wherein the surface regions have a larger radius of curvature than the pipe elements. When these coupling segments are initially positioned straddling the pipe ends, their surface regions initially contact pipe ends at a point about midway between opposite ends of each coupling segment. The surface regions near the ends of the segments are in spaced relation to the pipe element surface. When the fasteners are tightened, these coupling segments deform, and the surface regions on each segment progressively engage the outer surfaces of the pipe elements outwardly from the midpoint to each opposite end. Tightening the fasteners bends the coupling segments around the pipe ends by applying a bending moment. The moment arm of this bending moment extends between the leading edge of the surface region that is in contact with the pipe element on one side of the coupling segment, and the fastener on that one side. Initially the moment arm extends between the midpoint of the coupling segment and the fastener. As the surface region in contact with the pipe element grows progressively larger, the leading edge of this surface region moves from the midpoint of the segment toward the fastener. Consequently, the moment arm between leading edge and the fastener decreases, and the tension force in the fasteners required to deform the coupling segments increases as the coupling segments deform. The torque required to tighten the fasteners increases as a result. For large diameter couplings the required torque can exceed reasonable levels, causing difficulty for coupling of pipes using hand tools. It would be advantageous to have a deformable pipe coupling wherein the torque required to tighten the fastener and deform the coupling does not increase significantly as the coupling is deformed into contact with the pipe elements.